The heterostructure was first investigated in a conventional TEM (Philips
CM200) operated at room temperature at 80 kV. Fig. REF b
shows a bright field (BF) image of the freely suspended heterostructure.
At the locations marked with red arrows, dark patches of dirt are
clearly visible. Their nature and position on the sample are discussed
below. Fig. REF c shows the electron diffraction pattern
obtained by illuminating the whole suspended area of the sample. Two
distinct sets of diffraction spots with hexagonal symmetry and 1°
relative angular rotation can be observed. Because of the mismatch
between the lattice constants of graphene and hBN (1.8%, with hBN
being the larger of the two), it is possible to assign the outer and
the inner set of spots to the graphene and to the hBN lattice, respectively.
The green and the blue arrow in Fig. REF c indicate one graphene
and one hBN diffraction spot, respectively. The combination of lattice
mismatch and relative rotation is expected to produce a moirÃ© superlattice
as already observed by AFM and STM in other works for the case of
graphene on bulk hBN.[1]}, [2]}, [3]}, [4]}, [5]}, [6]}
The moirÃ© interference pattern can be conveniently visualized in the
TEM: Fig. REF d shows a dark field (DF) image of the same
area of Fig. REF b, acquired with a sample tilt of 16°.
Fig. REF e shows the area inside the yellow square in Fig. REF d
at higher magnification. The tilt axis in the reciprocal space is
indicated by the dotted black line in Fig. REF c and the
objective aperture used for DF imaging is marked by a red circle.
Note that the objective aperture was large enough to contain both
one graphene and one hBN spot. In contrast to the BF image, here a
strong modulation of the intensity appears, with bright spots arranged
in a triangular lattice. The periodicity of this modulation is 9.8
nm, very close to the predicted moirÃ© period of 9.9 nm for a 1°
misaligned graphene/hBN bilayer.[7]} Interestingly,
at some locations the moirÃ© interference pattern is completely suppressed
(marked by arrows, corresponding to the same locations marked in Fig. REF b).
We interpret these regions (which also appear much darker in the BF
image) as pockets of contamination trapped between the layers, as
reported previously on the basis of cross-sectional TEM imaging.[8]}
At these locations, the two layers are effectively separated by amorphous
contamination and the diffraction conditions for moirÃ© interference
are suppressed. Indeed, the presence of the moirÃ© in the DF-TEM images
indicates that most of the graphene/hBN interface is atomically clean.
Therefore, the clearly visible homogeneously distributed contamination,
similar to what is typically seen in TEM studies of graphene, must
be on the outer surfaces of the heterostructure.
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